1. Field of the Invention
The present invention relates to a time dependent degradation determination method for a radiation image detector and, more particularly, to a method of determining whether or not a setting error of an imaging plane of the radiation image detector is on the increase, and an apparatus for implementing the method.
2. Description of the Related Art
Panel type radiation image detectors are put into practical use. Such a detector includes an imaging plane having pixels, each storing charges by receiving radiation representing image information according to an amount of radiation received, disposed in a two-dimensional matrix and outputs image data representing the image information through a reading operation, as described, for example, in Japanese Unexamined Patent Publication No. 2006-156555. Generally, the pixels include a charge generation layer that generates charges by receiving radiation, a voltage application electrode for applying a voltage to the charge generation layer, a charge collection electrode for collecting the charges generated in the charge generation layer, and a switching element for reading out the charges collected by the charge collection electrode, which are formed, for example, of a TFT (thin film transistor) active matrix array.
Generally, the panel type radiation detectors described above are formed in a quadrangle, i.e., a rectangle or a square, and used widely to record transmitted radiation image information of a subject by emitting radiation transmitted through the subject. The quadrangularly shaped radiation image detector may sometimes be used for recording a long length image representing a long portion of a subject, such as the entire spine of a human body, in which the detector is shifted along a predetermined shift axis and receives radiation transmitted through the same subject at each shifted position.
When the panel type radiation detector is used in the manner as described above, a reading operation is performed with respect to each emission of radiation (radiation image recording) and image data representing a radiation image are obtained by each reading operation. Thereafter, these image data are combined together to obtain image data representing a long portion of a subject. As for the method of obtaining image data representing a long length radiation image in the manner as described above, for example, Japanese Unexamined Patent Publication No. 11 (1999)-244270 describes a method that uses, as an example, a cassette having a phosphor screen.
In the mean time, when combining radiation images in the manner as described above, a misalignment may sometimes occur at a joint of the combined image due to an inclination of the imaging plane of the panel type radiation image detector. There are several types of “inclination of imaging plane”, which will be described, hereinafter, in detail with reference to FIGS. 10 and 11.
A of FIG. 10 schematically illustrates a system for recording (imaging) a radiation image viewed from a side thereof. In the drawing, reference numeral 100 is a radiation source, reference numeral 101 is a stand for guiding a panel type quadrangular radiation image detector D, and the reference numeral 102 is the imaging plane of the radiation image detector D. In this example, it is assumed that grid 103 will be recorded as the subject in order to facilitate understanding of the problem. That is, imaging plane 102 of the radiation image detector D will be exposed by radiation 104 emitted from radiation source 100 and transmitted through grid 103.
In this case, the panel type quadrangular radiation image detector D is set such that the panel surface and one side of the panel become parallel to a direction in which stand 101 extends (arrow H direction) and shifted in the arrow H direction. That is, the arrow H direction is the shift axis in this case. The radiation image detector D, standing still before and after the shifting, is exposed to radiation 104 transmitted through grid 103, whereby first and second operations of radiation image taking are performed.
Here, one problem may arise that imaging plane 102 (two-dimensional pixel matrix constituting the imaging plane) is inclined by an angle of α with respect to the panel surface due to an assembly error of the radiation image detector D or the like and, if so, radiation images of grid 103 obtained by the first and second radiation emissions become like that shown in B and C of FIG. 10 respectively. That is, when joining a lower edge portion of first recorded image with an upper edge portion of second recorded image, the horizontal length of the subject differs between these portions and misalignment occurs at the joint.
In this case, the inclination angle α of the radiation image detector D with respect to the panel surface corresponds to the inclination angle thereof with respect to the shift axis H of the two-dimensional matrix since the radiation image detector D is set in the manner as described above.
Next, another problem will be described with reference to FIG. 11. A of FIG. 11 schematically illustrates a system for recording (imaging) a radiation image viewed from the front. In the drawing, reference numerals 101, 102, 103 are a stand, an imaging plane, and a grid respectively which are identical to those shown in A of FIG. 10. In this case, the radiation source is omitted in the drawing, but disposed such that radiation is emitted along an emission axis perpendicular to the plane of the drawing.
Also, in this case, the panel type quadrangular radiation image detector D is set in the same manner as in A of FIG. 10 and shifted in the arrow H direction, in which the radiation image detector D, standing still before and after the shifting, is exposed to radiation, whereby first and second operations of radiation image taking are performed.
Here, as another problem, two-dimensional pixel matrix may sometimes be inclined by an angle of γ with respect to one side of the panel in a plane parallel to the panel surface (plane parallel to the plane of the drawing) due to an assembly error of the radiation image detector D or the like. Note that some of the pixels are denoted by G in A of FIG. 11. If that is the case, radiation images of grid 103 obtained by the first and second radiation emissions become like that shown in B and C of FIG. 11 respectively. That is, when joining a lower edge portion of first recorded image with an upper edge portion of second recorded image, a fault slip like misalignment occurs at the joint.
In this case also, the inclination angle γ of the two-dimensional matrix with respect to one side of the panel corresponds to the inclination angle thereof with respect to the shift axis H of the two-dimensional matrix since the radiation image detector D is set in the manner as described above.
Taking the case, as an example, in which the size of the radiation image detector D is 40 cm×40 cm, the distance (SID) from the radiation source to the imaging plane is 180 cm, the misalignment at the joint of the combined image is about 0.5 mm when the inclination angle α is 0.31° or when inclination angle γ is 0.07°, which is significantly large.
So far, the problems of the case in which the panel type quadrangular radiation image detector is shifted accurately along the shift axis parallel to the surface and one side of the detector, and the two-dimensional matrix of pixels is inclined in the detector have been described. But, even in the case where the two-dimensional pixel matrix is not inclined in the radiation image detector, that is, the matrix is formed parallel to the surface and one side of the panel type quadrangular radiation image detector, if the radiation image detector itself is disposed inclined with respect to the detector shift axis, then the problems similar to those described above will naturally occur. Statuses when the radiation image detector itself is inclined in this way and the two-dimensional matrix is inclined with respect to detector shift axis by inclination angles α, γ are shown in FIGS. 12 and 13 respectively.
Here, the description has been made of a case in which the inclination of the matrix is constant even the radiation image detector is shifted. Where the radiation image detector is gradually inclined as it is shifted, the inclination of the matrix varies with the shifting of the radiation image detector. In such a case, a similar problem will occur. FIG. 14 schematically illustrates such situation, in which A of FIG. 14 schematically illustrates a system for recording (imaging) a radiation image viewed from the front. In the drawing, reference numerals 101, 102, 103 are a stand, an imaging plane, and a grid respectively which are identical to those shown in A of FIG. 10. Also in this case, the radiation source is omitted in the drawing, but disposed such that radiation is emitted along an emission axis perpendicular to the plane of the drawing.
In this case, a matrix inclination occurs as the radiation image detector is shifted and at the same time the matrix is shifted in a lateral direction. Such a phenomenon occurs due to, for example, low accuracy of a guide mechanism for guiding the movement of the radiation image detector, setting of a relatively large gap between, for example, a guide rod and a guide member of the guide mechanism, or the like.
At this time, radiation images of grid 103 obtained by the first and second radiation emissions become like that shown in B and C of FIG. 14 respectively. Also in this case, when joining a lower edge portion of first recorded image with an upper edge portion of second recorded image, a fault slip like misalignment occurs at the joint.
Further, the problem of misalignment at an image joint also occurs not only when the pixel matrix is inclined but also when the matrix is displaced from a predetermined position at the time of radiation emission. Hereinafter, the displacement will be described in detail.
FIG. 15 schematically illustrates the situation when such displacement occurred. A of FIG. 15 schematically illustrates a system for recording (imaging) a radiation image viewed from a side thereof. In the drawing, reference numeral 100 denotes a radiation source. Normally, when taking radiation images to be combined, the radiation image detector D is placed at positions at the time of first and second radiation emissions so as to overlap to a certain extent in the direction of shift axis H. For example, if aging deterioration has occurred in the moving mechanism of the radiation image detector D, however, the radiation image detector D may be displaced in a direction parallel to shift axis H from the predetermined position at each radiation emission. FIG. 15 illustrates an example case in which the radiation image detector D is displaced downward from the predetermined position by a length Δy at the second radiation emission.
At this time, radiation images of grid 103 obtained by the first and second radiation emissions become like that shown in B and C of FIG. 15 respectively. In this case, image combining is performed as the position in the first recorded image denoted by y0 in B of FIG. 15 would be matched with the upper edge of the second recorded image. In actuality, however, the upper edge of the second recorded image is displaced by the length Δy and, therefore, misalignment occurs at the image joint.
Further, such displacement as described above may sometimes occur in a direction orthogonal to shift axis H, as well as in a direction parallel to shift axis H. FIG. 16 schematically illustrates a situation in which such displacement has occurred. A of FIG. 16 schematically illustrates a system for recording (imaging) a radiation image viewed from the front. The radiation source is omitted in the drawing, but disposed such that radiation is emitted along an emission axis perpendicular to the plane of the drawing.
Normally, when taking radiation images to be combined, the radiation image detector D is placed at positions corresponding to each other in a direction orthogonal to shift axis H at the time of first and second radiation emissions. For example, if aging deterioration has occurred in the moving mechanism of the radiation image detector D or if stand 101 (more specifically, the rail for guiding the radiation image detector D) is curved as shown in the drawing, however, the radiation image detector D may be displaced in a direction orthogonal to shift axis H from the predetermined position at the time of radiation emission. FIG. 16 illustrates an example case in which the radiation image detector D is displaced to the right side from the predetermined position by a length Δx at the second radiation emission.
At this time, radiation images of grid 103 obtained by the first and second radiation emissions become like that shown in B and C of FIG. 16 respectively. In this case, image combining is performed as the positions of the first and second recorded images in the left/right direction, i.e., in a direction orthogonal to shift axis H would correspond to each other. In actuality, however, the second recorded image is displaced by the length Δx and, therefore, misalignment occurs at the image joint.
Aforementioned Japanese Unexamined Patent Publication No. 11 (1999)-244270 also describes a method which, when combining two images in the manner as described above, takes images of a grid provided in the cassette together with images of a subject and corrects the misalignment between the two images (in this case, the misalignment arising from the difference in distance between the subject and imaging plane) based on the grid images. But the method described in Japanese Unexamined Patent Publication No. 11 (1999)-244270 can not detect the inclination or displacement of the two-dimensional matrix that will occur when a panel type quadrangular radiation image detector is used. Therefore, it is obvious that the method can not correct misalignment between images based on such inclination or displacement.
Consequently, it is conceivable to detect the inclination or displacement of the two-dimensional matrix, which is a setting error of the imaging plane of a radiation image detector, and to perform correction processing on the two images to be combined for eliminating distortion based on the detected inclination or displacement. The inclination or displacement of the two-dimensional matrix, however, may increase due to aging deterioration, so that continued performance of the correction processing under the same condition without knowing this causes a problem that a misalignment occurs in an image joint even after the correction processing has been performed.
The present invention has been developed in view of the circumstances described above and it is an object of the present invention to provide a time dependent degradation determination method for a radiation image detector capable of clearly notifying a user that a setting error of the imaging plane of a radiation image detector has increased to an extent that exceeds an acceptable range.
It is a further object of the present invention to provide a time dependent degradation determination apparatus for a radiation image detector capable of implementing the time dependent degradation determination method for a radiation image detector described above.